Hiking Mount Humphreys of the San Francisco Peaks in Northern Arizona: Part II – My Geologic Ascent

Just 10 miles north of Flagstaff resides a spectacular edifice known as San Francisco Mountain in geological circles or simply “the Peaks” by the locals. It is the centerpiece of the San Francisco Volcanic Field and towers over the surrounding Colorado Plateau with its six-summits rising to an elevation between 11,000 and 13,000 feet. In fact, it’s the only alpine mountain in Arizona and the tallest in the state with the peak of Mount Humphreys rising to 12,633 feet. My plan was to climb Humphreys and take notes on the geology along the way.

This telephoto shot of the Peaks’ southern flank taken from my hotel in Flagstaff belies its massivity.
The angular summit on the left is Mount Agassiz which blocks our view of Mount Humphreys.

A Google Earth big picture
Seen from the west, San Francisco Mountain’s peaks and interconnecting ridgeline form a horseshoe-shaped ring. Within the Peak’s embrace lies a deep, elliptical central depression called the Inner Basin, the caldera of an ancient volcano. It leads to Lockett Meadow, and ultimately to the northeast breach where diminutive Sugarloaf Mountain stands guard, the last breath of the volcano to erupt. These landforms are the eroded remnants of a massive stratovolcano that erupted 2.78 million years ago. With typically steep flanks, a conical shape and a multi-layered architecture, it catastrophically met its demise between 250,000 and 400,000 years ago.

The Humphreys trailhead is located at the Snowbowl ski area’s car park. Notice the six peaks
that comprise San Francisco Mountain and its caldera, the Inner Basin.  

The final events are still debated by geologists, be they a vertical or sideways blast (alla Mount St. Helens) or a cataclysmic collapse into its own structural plumbing. Most investigators agree that a mechanism of collapse, subsidence or engulfment due to the withdrawal of magma from its magma chamber is responsible for the volcano's contemporary presentation rather than an evacuation outward. Either way, the volcano was reduced to a geothermally-extinct shell, exposed to the ravages of time and erosion. A multitude of Pleistocene alpine glaciers, Holocene gravitational flank collapse and debris flows left their marks on the ravaged stratocone.

Please visit my previous post Part I where I discuss the Peaks’ geo-morphology and geo-genesis in greater detail. 

“Big Picture” Stratigraphy

San Francisco Mountain (SFM hereafter) resides within the San Francisco Volcanic Field (SFVF hereafter) along with a plethora of volcaniforms. The entire field is situated near the southwestern boundary of the geomorphic province of the Colorado Plateau with the Basin and Range’s Transition Zone. Before we initiate our geological ascent, let’s review the volcano’s stratigraphy from the top down.

San Francisco Mountain stratigraphy

The conical shape and vertical stratification of SFM is attributable to the alternate layering of effusive and explosive eruptive materials of lava, pyroclastic debris and lahars (mudflows). SFM is considered to be an andesitic-dacitic stratovolcano built mostly by effusive activity that produced andesites (85%), dacites (12%) and rhyolites (1%). The andesites extruded from central vents fed from a magma reservoir; whereas, silicic lava tended to erupt from the volcano’s base and flanks. Magmas are generally plagioclase-dominated with products exhibiting magma-mixing.

A succession of older and younger andesites and dacites are thought to represent eruptive stages, four in all. “Older Andesite” lava flows constitute mainly the western part of the volcano (including Humphreys summit), while “Younger Andesites” are present on all flanks. Dacites are found on all slopes of the volcano but principally on the lower flanks. Both andesite and dacite are of intermediate mineralogical composition and are silica-rich which affects the volcano’s architecture and behavior. 

Simplified Geologic Map of San Francisco Mountain

Red outline marks the stratovolcano’s geologic boundary

(Karatson et al, 2010)

This geologic cross-section is through Humphreys and Fremont Peaks, two of SFM’s six peaks, and transects the caldera of the volcano. In the region of Humphreys, notice the layered Older and Younger Andesites (Qao and Qay) and Dacites (Qd and Qdo) that constitute the flanks of SFM (specifically an upper pyroxene andesite, a hornblende biotite dacite and a lower hypersthene dacite). On the floor of the Inner Basin are two parallel, resistant ridges called Core Ridge and Secondary Core Ridge and their dikes. They are remnants of the central conduits that fed the volcanic edifice. Radial dikes also fed flank eruptions. We’ll observe many of these structures on our climb of Humphreys.

Cross-section of the SFM through the Inner Basin from NNW to SSE
(Modified USGS map of SFM, Coconino County, Arizona by Richard F. Holm, 1988)

Here’s a link to a strat map of the SFM complex: http://ngmdb.usgs.gov/Prodesc/proddesc_9878.htm.

The San Francisco Volcanic Field

The SFVF (below) is a 4,800 square kilometer region decorated with over 600 Late Miocene to Holocene volcaniforms. It includes the Peaks and monogenetic (single eruption) cinder cones, lava domes, vents, dikes, and associated lava and pyroclastic flows. Volcanism both evolved and migrated on the field in an increasingly easterly direction with greater acceleration, increased magma production and eruption frequency. The field dips northeast at 1/2°-2° coincident with the planar surface of the Colorado Plateau. The field is predominantly basaltic; whereas, composition ranges from basalt to andesite to dacite and rhyolite.

Much can be said and remains to be learned about the field's enigmatic intraplate locale, its tectonic implications, its relationship to other late Cenozoic volcanism in this sector of the Colorado Plateau, and to the advancement of basin and range extension. Again, please visit my previous post Part I for elucidation.

Colorado Plateau stratigraphy

SFM rests on a bed of “older” basaltic flows from 10 to 4 Ma. The volcanic field overlies a mile-thick sequence of sedimentary Paleozoic (Cambrian through Middle Permian Kaibab Limestone) and Mesozoic (Early Triassic Moenkopi Formation) rocks of the Colorado Plateau. The Phanerozoic strata, in turn, unconformably overlie a Proterozoic crystalline basement complex. These layers are best seen within the Grand Canyon, only 45 miles away. Many geologists suspect the Grand Canyon to have formed within the last 6 million years, the time frame of the genesis of the SFVF. What a juxtaposition of geological activity!

Concurrent with volcanism on the plateau’s southern margin, normal faults that formed the Basin and Range Province in southern Arizona have encroached upon the plateau. In association with thinning of the crust, magma has found its way to the surface not only on the Grand Canyon’s North Rim but into the SFVF. Many geologists view the presence of faulting and volcanism as a clear indication that someday the Colorado Plateau will become an extension of the Basin and Range, regions that have already succumbed to extension.   

Schematic Cross-section beneath San Francisco Mountain
(From Morgan et al, 2004)

Mount Humphreys Trailhead

With temps in the upper 50’s, gray overcast skies, and concerns about lightning and lack of visibility at the summit, I was anxious to initiate my climb very early. I arrived at the mountain before sunrise after a short drive from Flagstaff on US 180. I suspect that many flatland-easterners such as myself think of Arizona as having mostly deserts, but there are half-dozen or so ski areas within the state, and actually 25 peaks over 10,000 feet!

Humphreys trailhead (black dots) is at the Arizona Snowbowl’s parking lot at the base of Agassiz’s western flank (35°19′52.61″ N, 111°42′41.73″ W). It crosses a ski trail and abruptly plunges into the Kachina Peaks Wilderness of the Coconino National Forest. After switchbacking its way to the Agassiz Saddle, it heads north to Humphreys across the cols that connect a few false peaks. The journey, considered strenuous by most accounts, is 4.8 miles with an elevation gain of 3,652 feet.

(Modified from Arizona Snowbowl’s Trail Map)



By the way, Mount Humphreys’ namesake was Andrew A. Humphreys, a profane and no-nonsense, war-loving Union Army Brigadier General and Chief of the U.S. Army Corps of Engineers that surveyed the region. SFM was named earlier in the 17^th century by Franciscan priests living at a nearby Hopi mission.


Mesa Butte Fault and its lava domes

Viewed at dawn from the car park, we see the closely-spaced lava domes of Bill Williams Mountain (far left, dated 4.2 to 3.6 Ma), Sitgreaves Mountain (left of center, dated 2.9 to 1.9 Ma), and Kendrick Peak (far right, dated 2.7 to 1.4 Ma). Further northeast along the fault lies Slate Mountain (1.5 Ma). Their silicic to intermediate rocks are viscous, silica-rich dacites, andesites and rhyolites. These volcaniforms mark the western and youngest section of the SFVF between 10 and 30 miles to the west.

The domes are aligned (see strat map below) on a northeast trend of the 150 km long Mesa Butte Fault, likely longer within the subsurface. This high-angle, normal fault resulted from extensional forces that concentrated volcanic vents along its course, the path of least resistance for the episodic ascent of rising magma. These fault systems of late Cenozoic age are related to ancient fracture systems at depth that transect a Proterozoic crystalline basement. They are viewed as indicative of the encroachment of extension on the plateau. Vent alignments along or parallel to these deep-seated crustal structural trends are common on the volcanic field and are often associated with basaltic cinder cones, dike injections and even silicic volcanoes.

Merriam's Life Zones
Ascending a mountain is analogous to traveling into increasingly northern latitudes as harsher and less tolerant growth-conditions for both flora and fauna are encountered. The idea that climatic gradients determine vegetative communities neither began nor ended with the biologist C. Hart Merriam in 1889. However, his concept of “life zones” that succeed each other with elevation was a milestone in the newly developing science of ecology. His research took him to the depths of the Grand Canyon and to the heights of the San Francisco Peaks which contain four of his six zones.

Merriam’s Life Zones (right) and their modern names (left) are labeled on the profile of the Peaks below. The elevation of the zones varies, since the north-facing slope is cooler and wetter than the south-facing slope. These zones can be extended to cover all the world's vegetation types with the addition of the tropical zone, and fluctuate over time in response to the dynamic nature of Earth’s climate.

 (Modified from cpluhna.nau.edu/Biota/elevational_range.htm)

Plunging in to a mixed conifer forest

After leaving the parking area, the trail skirts the base of a grass-covered ski trail before plunging into a tall, aromatic mixed-forest of Aspen, Ponderosa pine and Douglas fir at 9,375 feet, dark in the subdued morning light. We just entered the Mixed Conifer Forest of Merriam’s Canadian Zone. The lofty peaks of SFM generate their own weather with elevated precipitation and cooler temperatures compared to the surrounding semi-arid Colorado Plateau with its pinyon and juniper of Merriam’s Upper Sonoran Zone. We’re walking on a large, gravity-dispersed colluvial apron originating from the flanks of SFM.   

Switchbacks gauge one’s ascent
Once in the forest, the trail enters a series of five or six switchbacks that traverse Humphreys' steep slopes and large gullies, and serve as milestones to gauge one’s ascent. Only one visible outcrop was seen, but scattered within the forest numerous dark to medium-gray andesite boulders have weathered from outcrops undoubtedly from above. In addition, the outer slopes are an amalgamation of alluvium in all drainages, colluvium of silt, sand, pebbles and boulders, talus on the higher and steeper slopes, glacial till and outwash (larger outer gullies), and coarse, unsorted deposits of both avalanche debris and lahars. Blanket all of the above with a mixed forest and dense understory.

A stream of boulders

The first and third switchbacks abruptly reverse directions at a massive boulder stream, typical of glacial environments, that is rather difficult to negotiate. You can spot it on the Google Earth map above. The rock slide consists of unconsolidated boulders of andesite that have cascaded down the mountain’s flank likely facilitated by the movement of ice and a millennia of freeze-thaw cycles. Looking downslope to the west from out on the stream, a lone cinder cone on the volcanic field can be seen in the distance.

The third switchback

Upon gaining some elevation by the third switchback, I again ventured out onto the stream and was rewarded with a picture-perfect view of Sitgreaves (left) and Kendrick Mountains (right) to the west, similar to the perspective at the trailhead. Kendrick is the second highest volcano in the field at 10,418 feet. Much of its plant cover was burned in a devastating forest fire in the summer of 2000. One can only imagine the immense sound generated by this catastrophic avalanche of rock. Notice how Engleman Spruce is beginning to invade the stream from its periphery.

Mount Agassiz

After the final switchback, the trail headed upslope through more open timber with views of Mount Agassiz and its ski trails across Snowbowl Canyon, and the Agassiz Saddle high on the ridge. A five-minute hail storm had me concerned about the weather, but I pressed on and it abruptly abated. Above the treeline, notorious summer monsoons punish the peaks with lightning, fierce winds and rain. The temps can drop 40 degrees in minutes with snow possible even in summer. Climbers beware! At nearly 11,000 feet, this is the Spruce-Fir Forest region that Merriam called the Hudsonian Zone. Humphreys’ treeline is about 11,400 feet.

Agassiz is second in height to Humphreys at 12,356 feet. Named after the celebrated Swiss geologist, paleontologist and educator (1807-1873), one of his many areas of study was ice ages and glaciers that coincidentally sculpted the Peaks.

The Agassiz Saddle

We’ve reached the barren and exposed, wind-whipped Agassiz Saddle at 11,800 feet that connects the summits of Humphreys to the north (left) and Agassiz to the south (right). From here, the Weatherford Trail heads south to the summits of Fremont and Doyle. This is the jagged rim of the stratovolcano comprised of dark gray Older Andesites and some dacites. Standing atop the saddle, you can really appreciate the caldera’s massivity, peering into its depth and surveying the perimeter of the rim.

Although skiing is allowed on Agassiz, it is forbidden to hike above the treeline year round due to the federally-listed and ecologically-threatened, flowering groundsel Packera franciscana (also Senecio franciscana). Besides the talus slopes of Agassiz, Humphreys and the saddle, it is found nowhere else in the world. For all you botanists out there, this plant is a ragwort and a member of the sunflower family. Its future is uncertain in light of climate change predictions since there is little habitat available for the plant to migrate upward in a climate-warming scenario. We are about to enter the protected Arctic-Alpine Zone where hiking off trail is prohibited.

 

The Inner Basin

In the photo below, the 5 X 3 km caldera is 3,280 feet below the saddle. Its deep Inner Basin is bounded on three sides by the steep walls of the volcano's eroded inner flanks with its outlet blocked by the rhyolitic dome of Sugarloaf Mountain (SL) that erupted about 220,000 years ago, the youngest product subsequent to the stratovolcano’s andesitic-dacitic evolution. The central cavity is a subject of debate in regards to its formation during the active phase of volcanics and its subsequent erosion. 

The basin’s evergreen and aspen-carpeted floor has glaciated features such as cirqued-walls, a U-shaped valley, unsorted deposits of till and outwash, and moraines. It’s blanketed with unconsolidated, poorly-sorted volcaniclastic debris shed from the inner flanks via a combination of glacial erosion and mass wasting that coalesces toward the mouth of the Interior Valley. Fluvial contribution appears minimal save intermittent drainages. Springs and wells within the porous and permeable glacial deposits of the Inner Basin are important sources of water for the nearby city of Flagstaff located just south of the Peaks. 

The purplish-red color of the slope on the right is from the high concentration of scoria coming downslope from a parasitic cone that was once active of the flank of the main volcano. Both scoria and basalt are extrusive rocks and that take vesiculation to the extreme. Vesicles are a result of trapped gas within the melt at the time of solidification.





Core Ridge                                                                                                                                               Dominantly-andesitic Core Ridge (CR in the above photo) and its andesitic-dacitic dikes are remnants of the volcano’s conduit system and amongst the oldest rocks of the central complex. A linear Core Ridge divides the Inner Basin into two embayments and may have exerted control over glacial erosion after its exhumation, since two cirques and moraines are found north and south of the ridge. The ridge has experienced topographic inversion whereby it stands out in relief attributable to its differential resistance to erosion, largely glacial. It is said to be erosionally emergent. Some geologists have observed a coincidence of vent alignment and a linear, east-notheast-trend between Core Ridge, the Interior Valley) formed after the construction of the stratovolcano and before Sugarloaf), the Sugarloaf dome, O'Leary Peak and Strawberry Crater. That suggests that they formed under the influence of a common structural control and that the magmas may be closely related in genesis.  

  

Geologic Map of Humphreys Peak and the Inner Basin in the vicinity of Core Ridge

(Qao), Older Andesites; (Qay), Younger Andesites; (Qd) Older Dacites; (Qdo) Older dacites;
(Qs), Surficial Deposits; (Qcc), Andesites of Core Ridge; (Qdi, Qai), Dikes of Core Ridge.

The eastern flank of the San Francisco Volcanic Field

Beyond SFM in the haze (above photo) lies the eastern side of the geologically-recent SFVF. It contains numerous cinder cones and lava flows including the dacite-porphyry domes (240,000 and 170,000 years) of double-topped O’Leary Peak (OL) on the left and the scoria dome (SC) of Sunset Crater (about 1,000 years ago). The tan, unforested area of Bonito Park (BP) is an inter-conal basin consisting of lavas and cinders overlying outwash from SFM glaciation.

Mount Humphreys’ inner flank

In the photo below, looking north from the saddle, Mount Humphreys’ summit at 12,633 feet is about a mile away on the corner of the northwest rim. Notice its inner flanks cut in cross-section that possess layered lava flows, dozens in all, extrusive deposits of andesite, dacite, tuff and pumice. The eruptive deposits moved upward, outward and then downslope from the volcano’s former central vent, now-vanished with the explosion that evacuated the core.

One might assume that the evolution of the volcano’s conical shape is simple in that it forms via the successive layering of eruptive products. But in reality, many stratovolcanos are complex with convoluted histories that are challenging to unravel. This is the case with SFM with its cone-collapse, rebuilding, and even multiple vent locations. Its conical profile is the result of aggradation (eruption and emplacement of volcanic materials) and degradation (destructive processes of erosion, glaciation, gravity-driven avalanching and post-eruption mass wasting). Long-term erosion is climate-driven. Traditionally, volcanic cones are better preserved in arid, cool climates rather than humid, equatorial ones.

The inner flank stratigraphy

This close-up (below) of Humphrey’s glacially-cirqued, inner flank reveals lens-shaped cross-sections of dacite and andesite lava flows. Stratovolcanoes are also called “composite" volcanoes from the alternate layering of effusive and explosive deposits.  The internal structure and plumbing of the edifice was initially revealed when the volcano met its demise and later sculpted by three major Pleistocene glaciations that ended about 10,000 years ago and followed by extensive Holocene gravitational collapse.

I wasn’t the only creature enjoying the view!

Bristlecone pine of the Krummholz

In the upper reaches of Merriam’s Hudsonian Zone wind-twisted, climate-stunted Bristlecone pines reach an age far greater than any other single-living organism known, up to nearly 5,000 years. They grow so slowly that their small stature belies their true age. This region is also referred to as the Krummholz or “crooked-wood” zone, the transition zone to the alpine tundra. Bristlecones are well suited to the harsh conditions of cold, wind, low precipitation and short growing season at the treeline. They are under protection at many National Parks, where their existence is threatened by human trampling, fungal disease and pine beetles.

An igneous sampling from Humphreys

Just below Humphreys’ summit, I made this impromptu grouping of igneous rocks based on color, texture and grain size. Clockwise from the top, we have medium gray dacite, reddish-brown andesite, vesicular basalt, rhyolite, vesicular pumice and pumice again. Do you agree with my identification?

The alpine tundra of the Peaks

Merriam’s Timberline or Sub-Alpine Zone begins at about 12,000 feet. Above that, the only alpine tundra environment in Arizona is located on the Peaks within Merriam’s Arctic-Alpine Life Zone. The defining characteristic of a tundra is its lack of trees, a Finnish word meaning “treeless heights.” At first glance, the exposed summit of the tundra appears depauperate and barren, but it’s far from that. Though treeless, bitter cold, swept by incessant desiccating and abrading high winds, and bombarded by ultraviolet radiation, it sustains a stalwart population of low (prostrate) shrubs, mosses, grass-like sedges and lichens that are genetically adapted to the extremely harsh growing conditions.

The arctic tundra of high latitudes is ecologically synonymous with the alpine tundra of mountain tops. Plant survival adaptations include ground-hugging, waxy and hairy leaves, low nutritional requirements (the cold, thin soil slows decomposition and nutrient-cycling), and adventitious roots (allowing severed rhizomes in the unstable talus to regenerate a new plant rather than reproducing vegetatively).

Seen below, fragile and slow-growing tundra vegetation clings to life in isolated pockets amongst lichen and moss-encrusted rocky crevices and depressions of andesite cobbles and boulders near Humphreys’ summit.

A sign warns hikers to “STAY ON THE TRAIL” to prevent irreparable damage to the fragile tundra. Although protected, the plants may be threatened due to climate change, the inescapable challenge that we must all face.



Mount Humphreys summit

I reached the summit of Humphreys at 9:30 AM, four hours and 15 minutes from the trailhead with ample stops for photos along the way. The trail on the ridgeline crossed a few false peaks as a tease and at times was both difficult to find and negotiate in the loose cinders. I lost it a few times and had to backtrack, but with the summit in view, the destination was obvious. The top was slightly cool, perhaps about 50º F with only a slight wind and overcast skies. There was a brief interlude when the clouds parted allowing the sun to shine directly on top. I spent almost an hour checking out the spectacular view and the amazing geology.

Seen from its north side, this is the rubble-strewn peak of Humphreys capped with an Older Andesite flow with a K-Ar age of 0.43 ± 0.83 Ma.

Southwest view of a false summit
This view to the southwest looks back on a false summit. The trail follows the ridgeline.

South view of Mount Agassiz and its Saddle

The west flank of Fremont Peak on the south rim is on the top left. Its ridge leads to Agassiz, the angular summit to the right. The oxidized iron of the scoria-stained slope is the west end of the tail of Core Ridge which unites with the Agassiz Saddle on the west ridge. Notice the linear growth-pattern of the trees in the basin that follow drainages and talus slopes that have developed.

Mount Agassiz’s glaciated summit is in the background. In the foreground, Older Andesites on the summit of Humphreys are harbingers of protection from the elements for the hardy vegetation of the tundra.

The San Francisco Volcanic Field to the west

Looking west through the haze, we can see the three lava domes of Bill Williams, Sitgreaves and Kendrick Mountains on strike with the Mesa Butte fault on the west side of the volcanic field. Notice the loose, volcanic rubble scattered about. 

The volcano’s outer flanks

In this wide-angle photo looking downslope, the outer flanks of the volcano have eroded into valleys and gullies that lead to poorly sorted debris fans of cobbles and boulders. These fans are heavily vegetated and splay outward radially from every direction beyond the volcano's visible base. The provenance of the clasts within the fans is located in the lavas and pyroclastic deposits above the fans. This can be seen on the bedrock map above. Studies of the debris fans called planezes that surround the Peaks have led some geologists to theorize a dual-cone volcano. In addition, portions of the outer slopes bear the signature of glaciation in the form of till, outwash and moraines. The boulder stream is not the same one that we encountered on our ascent. From the summit to the planar surface of the plateau well beyond the base of the volcano it’s a 5,000 foot difference in elevation!

View to the east

Facing east along the summit-line of Aubineau and Reese on the crater’s north rim, the forested Inner Basin is off to the right (south). Directly beyond the peaks of the north rim the dual-topped cinder cone of O’Leary Peak is directly in the line of sight. To the south (right) of O’Leary, an array of cinder cones pepper the landscape including Sunset Crater, all on the eastern side of the volcanic field.



View to the north

To the north in the haze lies elongate Gray Mountain 35 miles away entering from the left (west), the monoclinal east limb of the Kaibab Upwarp. It is the surface manifestation of a Precambrian fault at depth that was reactivated during Laramide time into a massive domal uplift. Barely visible at the far left is the mist-shrouded North Rim of the Grand Canyon, 65 miles away. In the foreground are many cinder cones that delineate the north side of the SFVF including SP Crater with its barely visible lava flow.

In summation

Three liters of water and 7 ¾ hours later with ample time for photos and reflection on the summit, I arrived back at the trailhead. At the bottom, I stopped at the register where I first signed in. Forty-four names had been added to the list since my start at sunrise. A busy day for all on the Humphreys Trail.